Method of manufacturing catalyst for purifying exhaust gas

ABSTRACT

A method of manufacturing a catalyst for purifying exhaust gas, the catalyst including a pentasil-type crystalline aluminosilicate catalyst carrier carrying thereon a copper component and a phosphorus component. The method is characterized by adding the catalyst carrier into a solution containing a copper component and a phosphorus component, or a buffer and having a predetermined pH adjusted with ammonia, and subsequently adding an acid to the solution so as to adjust the pH of the solution to 7.0 or less, to thereby incorporate the copper component and the phosphorus component into the catalyst carrier. According to the present invention, a catalyst for removing exhaust gases which is durable and which has a high NO X  removal factor can be prepared in a stable manner.

This application is a 371 of PCT/JP96/03471, filed on Nov. 27, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing a catalystparticularly useful for removing nitrogen oxides exhausted fromtransportable internal-combustion engines such as those used ingasoline-fueled automobiles and diesel-powered automobiles; stationaryinternal-combustion engines such as those used in cogeneration plants;combustors such as those used in boilers; furnaces of plants; etc., byreducing the nitrogen oxides to harmless gases.

2. Background Art

Generally, the exhaust gas exhausted from transportable or stationaryinternal-combustion engines, combustors, or furnaces of plants containsa large amount of nitrogen oxides (NO_(X)) such as NO or NO₂. Thesenitrogen oxides (NO_(X)) are considered to cause not only photochemicalsmog but also damage to the human respiratory system.

Conventionally, there has widely been known a method for decreasing theNO_(X) content in exhaust gas, wherein NO_(X) is removed throughreduction, by use of a catalyst, with carbon monoxide or hydrocarbonscontained in the exhaust gas.

Catalysts commonly used for such purposes include those prepared byincorporating metals, by ion-exchange, impregnation, etc., into acarrier formed of zeolite such as crystalline aluminosilicate.

In particular, even under high gas hourly space velocity (GHSV)conditions, a crystalline aluminosilicate catalyst carrying copper as ametal component can effectively remove nitrogen oxides (NO_(X)) from theexhaust gas containing a large amount of oxygen, by use of hydrocarbonas a reducing agent. Therefore, this type of catalyst is considered tobe a promising catalyst for purifying exhaust gases from transportableor stationary internal-combustion engines.

However, the copper-on-crystalline aluminosilicate catalyst has poordurability to heat and steam; i.e., the valence, oxidation state, anddispersion state of copper incorporated in the catalyst are subject tochange when the catalyst undergoes treatment with heat and steam. Thus,there remains a drawback that constant purification performance overprolonged periods is not obtained at temperatures of 600° C. or more orwhen the exhaust gas contains a large amount of moisture.

To overcome this drawback, in Japanese Patent Application Laid-Open(kokai) No. 6-134314 the present inventors disclosed, as a catalyst forpurifying exhaust gas and having remarkably enhanced durability, acatalyst comprising a pentasil-type crystalline aluminosilicate catalystcarrier carrying thereon a copper component and a phosphorus component.

With an ion-exchange method, which is suited for homogeneousincorporation of the copper component and the phosphorus component, thepH ranges of mother solutions which are proper for incorporation ofrespective components in required amounts differ from each other.Consequently, the pH range where both components are simultaneouslyincorporated in proper amounts is too narrow to be satisfactorilymaintained by industrial apparatuses.

Although respective components may sequentially be incorporated withinpH ranges suited for the respective components, an increased number ofincorporation steps is not industrially advantageous at all.

SUMMARY OF THE INVENTION

The present invention was made in view of the foregoing, and an objectof the present invention is to provide a method of manufacturing, withina wide pH range and in a stable manner, a catalyst for purifying exhaustgas, the catalyst having excellent durability, containing a coppercomponent and a phosphorus component, and being suited for purifyingexhaust gas with high efficiency.

The present inventors have conducted earnest studies, and have foundthat the above object is effectively attained by elevating the pH of asolution used for incorporation of the copper component and thephosphorus component during an initial stage of incorporation andlowering the pH in the course of the incorporation step. The presentinvention was accomplished based on this finding.

Accordingly, the present invention provides the following.

(1) A method of manufacturing a catalyst for purifying exhaust gas, thecatalyst comprising a pentasil-type crystalline aluminosilicate catalystcarrier carrying thereon at least a copper component and a phosphoruscomponent, the method being characterized by adding the catalyst carrierinto a solution containing at least a copper component, a phosphoruscomponent, ammonia, and a buffer and having a pH of 8.0 or more, andsubsequently adding an acid to the solution so as to adjust the pH ofthe solution to 7.0 or less, to thereby incorporate at least the coppercomponent and the phosphorus component into the catalyst carrier.

(2) A method of manufacturing a catalyst for purifying exhaust gas asdescribed in (1), wherein the pH of the solution before the catalystcarrier is added is 8.0-12.0.

(3) A method of manufacturing a catalyst for purifying exhaust gas, thecatalyst comprising a pentasil-type crystalline aluminosilicate catalystcarrier carrying thereon at least a copper component and a phosphoruscomponent, the method being characterized by adding the catalyst carrierinto a solution containing at least a copper component, a phosphoruscomponent, and ammonia, and having a pH of 9.0 or more, and subsequentlyadding an acid to the resultant solution so as to adjust the pH of thesolution to 7.0 or less, to thereby incorporate at least the coppercomponent and the phosphorus component into the catalyst carrier.

(4) A method of manufacturing a catalyst for purifying exhaust gas asdescribed in (1), wherein the pH of the solution before the catalystcarrier is added is 9.0-12.0.

(5) A method of manufacturing a catalyst for purifying exhaust gas asdescribed in any one of (1) through (4), wherein pentasil-typecrystalline aluminosilicate has an MFI structure.

(6) A method of manufacturing a catalyst for purifying exhaust gas asdescribed in (1) or (2), wherein the buffer is at least one speciesselected from among ammonium nitrate, ammonium chloride, ammoniumacetate, and sodium acetate.

(7) A method of manufacturing a catalyst for purifying exhaust gas asdescribed in any one of (1) through (6), wherein the acid is at leastone species selected from among nitric acid, hydrochloric acid, andsulfuric acid.

PREFERRED EMBODIMENTS OF THE INVENTION

The preferred embodiments of the present invention will next bedescribed in detail.

Firstly, the catalyst for purifying exhaust gas manufactured by thepresent invention will be described.

In the present invention, a catalyst for purifying exhaust gas is formedby incorporating a specific component into a catalyst carrier comprisingpentasil-type crystalline aluminosilicate. Pentasil-type crystallinealuminosilicate is used as a catalyst carrier in the present invention,since crystalline aluminosilicates other than pentasil-type crystallinealuminosilicate have inherently poor hydrothermal durability, anddurability against steaming decreases further when a phosphoruscomponent is incorporated, thus such crystalline aluminosilicates arenot suited for a catalyst carrier requiring long-term stability.

In the present invention, pentasil-type crystalline aluminosilicaterefers to zeolite whose structural unit comprises an oxygen 5-memberedring, and examples thereof include ferrierite, mordenite, and ZSM-5 andZSM-11 having an MFI structure. As used herein, the MFI structure refersto ZSM-5 or a structure similar to ZSM-5, and examples categorized asthe MFI structure include ZSM-8, ZSM-11, ζ-1, ζ-3, Nu-4, Nu-5, TZ-1,TPZ-1, ISI-3, ISI-5, and AZ-1. These pentasil-type crystallinealuminosilicates may be respectively prepared through a known method.

Among these pentasil-type crystalline aluminosilicates, analumnosilicate having an MFI structure is preferred in view of excellenthydrothermal durability, with ZSM-5 synthesized by use of mordenite as aseed crystal being particularly preferred. Also, among theabove-described pentasil-type crystalline aluminosilicates, analuminosilicate having a mole ratio (SiO₂ /Al₂ O₃) of 10-200 ispreferred. When the mole ratio is less than 10, the hydrothermaldurability of zeolite itself is low to thereby induce reduction oflong-term stability of the corresponding catalyst, whereas when the moleratio is in excess of 200, ion-exchanging capacity is limited to reducethe amount of active metal to be incorporated, resulting in poorcatalyst activity. In the present invention, these pentasil-typecrystalline aluminosilicates may be used singly or in combination of twoor more species to form a catalyst carrier.

To the above-described pentasil-type crystalline aluminosilicates, theremay be added a variety of compounds such as an oxide used forconventional catalyst carriers to form a catalyst carrier, so long asthe compounds do not inhibit characteristics of the catalyst, such asperformance for purifying exhaust gas. For example, silica, alumina,silica-alumina, magnesia, or zirconia may be incorporated to enhance thedispersibility of the copper component and the phosphorus component.

In the catalyst for purifying exhaust gas of the present invention, atleast a copper component and a phosphorus component are incorporated inthe above-described catalyst carrier.

In the present invention, the amount of the copper componentincorporated into the catalyst carrier is preferably 0.8-30.0 wt. %,particularly preferably 2.0-15.0 wt. %, based on the total weight of thecatalyst as reduced to CuO. When the content is less than 0.8 wt. %, thecatalyst exhibits poor activity, due to the low copper content, whereaswhen the content is in excess of 30.0 wt. %, copper oxide (CuO), forexample, aggregates on the surface of the catalyst carrier to plug upmicropores of the catalyst carrier, which may sometimes cause reductionof the catalyst activity.

The amount of the phosphorus component incorporated is preferably0.1-5.0 wt. %, particularly preferably 0.1-2.0 wt. %, based on the totalweight of the catalyst as reduced to P₂ O₅. When the content is lessthan 0.1 wt. %, the stability of the copper component is not fullyensured to thereby sometimes result in failure to obtain a catalystexhibiting excellent hydrothermal durability, whereas when the contentis in excess of 5.0 wt. %, improvement of the durability commensuratewith the amount of incorporation is not obtained.

In the present invention, there may be incorporated components otherthan the copper component and phosphorus component so long as they donot adversely affect the catalyst activity and durability. For example,a metal component such as Co, Fe, Ga, or In may be incorporated in orderto further enhance the catalyst activity. A metal component such as rareearth metal, alkaline earth metal or Zr, or halogen compound thereof maybe incorporated in order to further enhance the durability of thecatalyst. Such other components are incorporated typically in an amountof approximately 0.05-10 wt. % based on the total amount of thecatalyst.

Next, a method of manufacturing a catalyst for purifying exhaust gaswill be described.

In the first stage of the method of manufacturing a catalyst forpurifying exhaust gas of the present invention, there is prepared asolution containing a copper compound and a phosphorus compound, whichcorrespond to the copper component and the phosphorus component to beincorporated in the catalyst carrier.

As the solvent for preparing the solution for incorporation, there maybe used solvents conventionally employed for preparation of a catalystby, e.g., incorporating an active component into a catalyst carrier, andthey may suitably be selected from among polar solvents such as water oralcohol (e.g., methanol, ethanol). Of these, water is preferably used inpractice.

No particular limitation is imposed on the copper compound, and theremay be used copper compounds such as inorganic acid salts, halides,organic acid salts, and complexes. Examples include inorganic acid saltssuch as copper nitrate or copper carbonate; halides such as copperfluoride, copper chloride, or copper bromide; organic acid salts such ascopper acetate or copper oxalate; and complexes such as a copper amminecomplex or a copper cyano complex. Of these copper compounds, inorganicacid salts are preferred in that they are easily incorporatedhomogeneously into the catalyst carrier, with copper nitrate beingparticularly preferred. These copper compounds may be used singly or incombination of two or more species.

No particular limitation is imposed on the phosphorus compound so longas it contains phosphorus and has solubility or compatibility to asolvent used, and there may be used phosphorus compounds such asinorganic phosphoric acids or salts thereof, or phosphoruschalcogenides. Examples of the inorganic phosphoric acids include (a)phosphoric acids having a variety of oxidation numbers of phosphorussuch as orthophosphoric acid, metaphosphoric acid, hypophosphoric acid,phosphorous acid, or hypophosphorous acid; and (b) condensed phosphoricacids such as polyphosphoric acids--e.g., orthopyrophosphoric acid,metapyrophosphoric acid, tripolyphosphoric acid, and tetrapolyphosphoricacid--and polymetaphosphoric acids--e.g., trimetaphosphoric acid,tetrametaphosphoric acid, and hexametaphosphoric acid. Examples of thesalts of the above-described inorganic phosphoric acids include alkalimetal salts such as lithium salts, sodium salts, and potassium salts;and ammonium salts. These salts also include hydrogen salts such asdihydrogen alkali metal orthophosphates, monohydrogen alkali metalorthophosphate, dihydrogen ammonium orthophosphate, monohydrogenammonium orthophosphate, monohydrogen alkali metal phosphite, andmonohydrogen ammonium phosphite, as well as normal salts such astrialkali metal orthophosphates and triammonium orthophosphate. Examplesof the phosphorus chalcogenide include phosphorus pentoxide, phosphorustrioxide, and phosphorus pentasulfide. Of these phosphorus compounds,lithium phosphates, sodium phosphates, and ammonium phosphates arepreferred in that a catalyst having excellent heat resistance is easilyobtained therefrom, with dihydrogen ammonium orthophosphate beingparticularly preferred. These phosphorus compounds may be used singly orin combination of two or more species.

The amount of the copper compound and that of the phosphorus compoundcontained in the solution for incorporation are not univocallyspecified, since they vary according to the amounts of respectivecomponents incorporated in the catalyst carrier or to conditions ofincorporation such as temperature or pH of the solution. Typically, theamount is preferably 0.2-2.0 mmol/g-cat for the copper compound and0.01-1.0 mmol/g-cat for the phosphorus compound. When the amount of thecopper compound and that of the phosphorus compound are selected to be2.0 mmol/g-cat or more and 1.0 mmol/g-cat or more, respectively, thecopper component or the phosphorus component aggregates on the surfaceof the catalyst carrier to sometimes result in failure to obtain acatalyst having sufficient exhaust gas-purification ability. Incontrast, when the amounts are selected to be 0.2 mmol/g-cat or less and0.01 mmol/g-cat or less, respectively, the copper component or thephosphorus component is not incorporated or only a small amount of thecomponent is incorporated through one incorporation operation, torequire many repetitions of the incorporation operation, which is notpractical.

When there is incorporated another component other than the coppercomponent and the phosphorus component, a compound that contains thecomponent and dissolves in a solvent to be used is appropriately chosen,and the compound is preferably dissolved in a solution containing theabove copper component and phosphorus component for simultaneousincorporation, i.e., one-step incorporation operation. Alternatively, acustomary incorporation treatment may be conducted separately before andafter the incorporation of the copper component and the phosphoruscomponent conducted in the present invention.

In the present invention, the pH of the solution is adjusted by addingammonia after a buffer (or no buffer) is added to the solution.

The purpose of adjusting the pH is to form a sufficient amount of acopper complex in the solution. The pH is adjusted by use of ammonia,since a copper ammine complex formed is suitable for achieving ahomogeneous incorporation of the copper component into the catalystcarrier in view of the ionic state, form, and size of the complex.

No particular limitation is imposed on the ammonia added, andcommercially available aqueous ammonia or a solution containing ammoniais appropriately used as undiluted or diluted with a solvent used in thesolution for incorporation. Generally, an ammonia source containingammonia in an amount of 0.5 vol. % or more is preferably used. When thesource having an ammonia content of less than 0.5 vol. % is used, alarge amount of an ammonia solution must be added to adjust the pH, andthe concentration of the copper component or the phosphorus component inthe solution for incorporation considerably varies to adversely affectincorporation at the desired concentration.

In the embodiments of the present invention, the pH of the solution tobe adjusted varies depending on whether the pH adjustment is effected byuse of the below-described buffer or without use of the buffer.

That is, when no buffer is added, the pH is adjusted to be 9.0 or more,preferably 9.0-12.0, whereas when the buffer is added in advance, the pHis adjusted to be 8.0 or more, preferably 8.0-12.0. When the pH isadjusted to be less than 9.0 for the former case and less than 8.0 forthe latter case, respectively, a sufficient amount of a copper amminecomplex is not formed. Therefore, the copper component is nothomogeneously incorporated in the catalyst carrier to result indeterioration of performance of the catalyst in purifying exhaust gas.

As mentioned above, the pH of the solution is adjusted after addition ofa buffer in one embodiment of the present invention, since the copperammine complex may be formed in a sufficient amount at the lower pHthrough addition of the buffer, and a phosphorus compound having highsolubility is effectively incorporated into the catalyst carrier.

No particular limitation is imposed on the buffer used in the presentinvention, and commercially available buffers may be used appropriately.A buffer adjusting pH to 7.0 or more is preferred, since ammonia must beadded in an amount larger than that required to adjust the pH of thesolution to 8.0 or more when there is used a buffer adjusting pH to 7.0or less. Examples of preferable buffers include ammonium nitrate,ammonium chloride, ammonium acetate, and sodium acetate. Of these,particularly preferred is a salt containing an ionic fragment which isalso a fragment of the copper compound and the phosphorus compound usedfor preparing a solution containing the copper component and thephosphorus component. For example, when copper nitrate is used as thecopper compound and dihydrogen ammonium phosphate is used as thephosphorus compound, ammonium nitrate is the most preferred buffer. Theamount of the buffer used, depending on the kind of the buffer, istypically 2-6 mol/mol-Cu ions.

In the present invention, at least a copper component and a phosphoruscomponent are incorporated into a catalyst carrier containingpentasil-type crystalline aluminosilicate, by use of the above-describedpH-adjusted solution for incorporation. The incorporation method willnext be described in detail.

With regard to the incorporation method of the present invention, acatalyst carrier containing pentasil-type crystalline aluminosilicate isadded into the above-described solution for incorporation that is heatedor cooled in a temperature range of 10-50° C., preferably 15-40° C., andthe mixture is allowed to stand or is preferably stirred for 1-10 hours,preferably 1-5 hours. The copper component or the phosphorus componentis ion-exchanged with cations of the catalyst carrier through the step.No particular limitation is imposed on the apparatus utilized forheating, cooling, or stirring, and customarily utilized apparatus may beutilized in the present invention.

After the treatment for the above-described time, pH of the solution isadjusted to 7.0 or less, preferably 6.5-4.0, by adding an acid to thesolution. The reason for adjusting pH of the solution to 7.0 or less isto incorporate into the catalyst carrier the ion-unexchanged coppercomponent or phosphorus component contained in the solution before pHadjustment.

No particular limitation is imposed on the acid for adjusting pH of thesolution, and nitric acid, hydrochloric acid, or sulfuric acid may beused in the present invention. Of these, nitric acid is preferred inthat a catalyst having excellent durability may be easily obtained.

Addition of the acid is preferably conducted at a pH descending rate ofapproximately 0.2-2/minute. When the rate is two or more, a coppercomponent is easily incorporated into the surface of a catalyst in theform of aggregates to result in failure to obtain a catalyst exhibitingsufficient performance for purifying exhaust gas, whereas when the rateis 0.2 or less, the effect for preventing aggregation commensurate withthe time consumed is not obtained.

The acid is added preferably under stirring in order to prevent theoccurrence of a sudden local pH change in the solution.

In the present invention, the solution is further allowed to stand orstirred for 0.5-4 hours, preferably 0.5-2 hours with heating or coolingin a temperature range of 10-50° C., more preferably 15-40° C., afteraddition of the acid. The copper component or the phosphorus componentcontained in the solution is incorporated into the catalyst carrier in adesired amount through this step.

In the present invention, after incorporation of the copper componentand the phosphorus component, the solution is filtered to recover asolid, which is washed and dried through a customary method. Forexample, filtration is typically conducted by use of a Buchner funnel, apressure filtration apparatus, a filter press, etc., and the solid isdried in air at 100-200° C. for 6-24 hours. In the present invention, adesired amount of the copper component and the phosphorus component maytypically be incorporated into the catalyst carrier through one step ofincorporation, or through repeated steps of incorporation in order torealize further homogeneous incorporation of the copper component andthe phosphorus component.

The thus-formed catalyst carrier carrying a desired amount of the coppercomponent and a desired amount of the phosphorus component is firedthrough a customary method to thereby obtain a catalyst. For example,firing is conducted by use of an apparatus conventionally used forfiring a catalyst, such as a muffle furnace or a rotary kiln, in air at500-700° C. for 0.5-8 hours.

In the present invention, the fired catalyst may further be treated witha substance such as an acid, an alkali, steam, ammonia, a halogen, orother nonmetallic compound or with heat to provide a catalyst used inthe present invention.

The obtained catalyst is normally in the form of powder and it may beused as is. The catalyst is preferably used after being formed into anarbitrary shape such as a spherical, columnar, star-like, or honeycombshape through use of a binder such as silica, alumina, silica-alumina,magnesia, or zirconia depending on uses. Alternatively, the catalyst ofthe present invention may also be used by application of powder thereofon separately made carrier substrates having a variety of shapes. Noparticular limitation is imposed on the material used as such carriersubstrates, and a variety of heat-resisting materials such as ceramicsand metals may be selected and used. For example, as a catalyst forexhaust gas from automobiles, there is preferably used a catalystcomprising ceramics having excellent strength, especially at hightemperatures, and long-term heat resistance, inter alia ahoneycomb-shaped carrier substrate made of cordierite, coated with apowder of the catalyst of the present invention.

EXAMPLES

The present invention will next be described in detail by way ofexamples, which should not be construed as limiting the invention.

Example 1

Three solutions were prepared: a solution comprising aluminum sulfatedodecahydrate (337.5 g), 97% sulfuric acid (362.5 g), and water (8,250g)(referred to as solution I); a solution comprising water glass (SiO₂ :28.5%, Na₂ O 9.5%, water 62%) (5,275 g) and water (5,000 g) (referred toas solution II); and a solution comprising sodium chloride (987.5 g) andwater (2,300 g)(referred to as solution III).

Then, solution I and solution II were mixed while simultaneously beingadded dropwise into solution III. The pH of the raw material mixture wasadjusted to 9.5 and mordenite (SiO₂ /Al₂ O₃) (mole ratio=2.0) (12.5 g)was added as seed crystals.

The raw material mixture was then placed in a 25-liter-autoclave, andstirred at 170° C. and 3,000 rpm under a sealed condition over 20 hours.After cooling, the reaction mixture was filtered to obtain aprecipitate, which was thoroughly washed with pure water and dried at120° C. for 20 hours to thereby synthesize crystalline pentasil-typealuminosilicate having ZSM-5 structure (MFI structure). Measurementresults from powder X-ray diffraction of this aluminosilicate are shownin Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Interplanar                                                                             Relative    Interplanar                                                                             Relative                                        distance (d) intensity distance (d) intensity                               ______________________________________                                        11.5      strong      4.65      strong                                          10.3 strong 4.28 weak                                                         9.1 weak 3.86 very strong                                                     7.5 weak 3.75 strong                                                          7.3 weak 3.46 weak                                                            6.5 weak 3.06 weak                                                            6.10 weak 2.99 weak                                                           5.64 weak 2.96 weak                                                           5.10 weak 2.00 weak                                                         ______________________________________                                    

The ratio SiO₂ /Al₂ O₃ (mole ratio) of this aluminosilicate was 32. Thisaluminosilicate was then fired at 550° C. in an air stream for 6 hours.

A solution for incorporation containing a copper component and aphosphorus component was then prepared as follows: copper nitratetrihydrate (132.5 g), dihydrogen ammonium phosphate (11.17 g), andammonium nitrate (178 g) were successively added into water (2,080 g)and 5% aqueous ammonia was added to the solution while solutiontemperature was controlled at 30° C. to adjust pH to 8.5. To theresultant solution, the above-described aluminosilicate (500 g) wasadded and the mixture was allowed to undergo ion-exchange for four hourswhile solution temperature was controlled at 30° C. Subsequently, 30%nitric acid was added to the solution to adjust pH to 6.0, and themixture was allowed to undergo ion-exchange for an additional one hour.The resultant slurry solution was filtered to recover a solid, which waswashed with water, dried at 120° C. for 24 hours, and fired at 500° C.for four hours to thereby obtain a target catalyst. The copper contentof the catalyst as reduced to CuO was 7.5 wt. % and the phosphoruscontent thereof as reduced to P₂ O₅ was 1.3 wt. %.

Next, performance for reduction-removing No_(X) of the above-describedcatalyst was evaluated based on the below-described initial activity andactivity after steaming treatment (hydrothermal treatment).

(1) Evaluation of Initial Activity

A sample of the obtained catalyst (2 cc) was introduced into a reactiontube made of stainless steel, and the tube was maintained attemperatures described below. A model gas was passed through theabove-described reaction tube as a gas to be treated at GHSV=80,000 h⁻¹.The model gas comprised NO_(X) (500 ppm), O₂ (6.0%), C₃ H₆ /C₃ H₈ (C₃ H₆/C₃ H₈ =2 (about 2,500 ppm as THC concentration)), and the balancenitrogen. The THC (total hydrocarbon) concentration refers to aconcentration of hydrocarbons as reduced to methane. Then, the gaseffused from the outlet of the reaction tube was introduced to achemiluminescence analyzer to measure NO_(X) content of the gas. Aremoval factor of NO_(X) after catalytic reaction was calculated bycomparison of the thus-obtained NO_(X) content to concentration ofNO_(X) in the model gas before introduction to the reaction tube. Theremoval factor of NO_(X) was evaluated at temperatures of the reactiontube of 300° C., 350° C., and 400° C., respectively. The results areshown in Table 2.

(2) Evaluation of Activity After Steaming Treatment (hydrothermaltreatment)

A sample of the catalyst prepared in Examples of the present inventionwas introduced into a reaction tube made of quartz, and the tube wasmaintained at 750° C. A nitrogen gas containing C₃ H₆ /C₃ H₈ (C₃ H₆ /C₃H₈ =2 (about 2,500 ppm as THC concentration)), O₂ (0.5%), and water (10vol. %) was introduced into the tube at GHSV=80,000 h⁻¹ over 16 hours toconduct steaming treatment.

After cooling the reaction tube, the catalyst recovered from the tubewas introduced into a reaction tube made of stainless steel, and removalfactor of NO_(X) to the model gas was evaluated under the conditionsidentical to those at the above-described evaluation of initialactivity. The results are shown in Table 2.

Example 2

The procedure of Example 1 was conducted through use of 5% aqueousammonia instead of ammonium nitrate, to adjust pH of the solution to11.5 and thereby prepare a catalyst of Example 2. The copper content ofthe catalyst as reduced to CuO was 6.7 wt. % and the phosphorus contentthereof as reduced to P₂ O₅ was 1.1 wt. %.

Example 3

The procedure of Example 1 was conducted through use of ammoniumcarbonate instead of ammonium nitrate, to thereby prepare a catalyst ofExample 3. The copper content of the catalyst as reduced to CuO was 6.0wt. % and the phosphorus content thereof as reduced to P₂ O₅ was 1.2 wt.%.

Example 4

The procedure of Example 1 was conducted through use of ammoniumpyrophosphate instead of dihydrogen ammonium phosphate, to therebyprepare a catalyst of Example 4. The copper content of the catalyst asreduced to CuO was 7.3 wt. % and the phosphorus content thereof asreduced to P₂ O₅ was 0.8 wt. %.

Comparative Example 1

The procedure of Example 2 was conducted except that adjustment of pH ofthe solution through use of 5% aqueous ammonia (pH of the solution, 2.5)and that through use of nitric acid were omitted, to thereby prepare acatalyst of Comparative Example 1. The copper content of the catalyst asreduced to CuO was 2.6 wt. % and the phosphorus content thereof asreduced to P₂ O₅ was 0.2 wt. %.

Comparative Example 2

The procedure of Example 2 was conducted through use of 5% aqueousammonia to adjust pH of the solution to 6.0, to thereby prepare acatalyst of Comparative Example 2. The copper content of the catalyst asreduced to CuO was 6.0 wt. % and the phosphorus content thereof asreduced to P₂ O₅ was 1.0 wt. %.

Comparative Example 3

The procedure of Example 2 was conducted except that adjustment of pH ofthe solution through use of nitric acid was omitted, to thereby preparea catalyst of Comparative Example 3. The copper content of the catalystas reduced to CuO was 3.2 wt. % and the phosphorus content thereof asreduced to P₂ O₅ was 0.01 wt. %.

Initial activity and activity after steaming (hydrothermal) treatmentwere evaluated for catalysts of Examples 2 to 4 and Comparative Examples1 to 3 under conditions identical to those of Example 1. The results areshown in Table 2.

                  TABLE 2                                                         ______________________________________                                                                Activity after                                          Catalyst Initial activity hydrothermal treatment                            No.     300° C.                                                                        350° C.                                                                        400° C.                                                                      300° C.                                                                      350° C.                                                                       400° C.                     ______________________________________                                        Ex. 1   57      86      82    30    68     78                                   Ex. 2 50 81 81 25 61 75                                                       Ex. 3 44 77 80 20 58 69                                                       Ex. 4 54 84 81 15 54 65                                                       Comp. 38 60 68  2 13 20                                                       Ex. 1                                                                         Comp. 22 30 43  1 15 21                                                       Ex. 2                                                                         Comp. 34 65 70  1 18 30                                                       Ex. 3                                                                       ______________________________________                                    

As shown in Table 2, the catalysts of the present invention achieve highremoval factors of NO_(X) even after steaming treatment and consequentlyexhibit excellent durability. Also, Table 2 shows extremely low removalfactors of NO_(X) after steaming treatment for Comparative Example 1, inwhich pH of the solution is not adjusted to result in almost noformation of a copper ammine complex and no substantial ion-exchange viacopper aqua ions. Similar to the case of Comparative Example 1,extremely low removal factors of NO_(X) after steaming treatment ascompared with that of the present invention are obtained for ComparativeExample 2, in which pH of the solution is adjusted to as low as 6.0 toresult in poor formation of a copper ammine complex. Moreover, extremelylow removal factors of NO_(X) after steaming treatment are obtained forComparative Example 3, in which pH of the solution is not adjusted byuse of an acid to result in almost no incorporation of a phosphoruscomponent.

As described above, the method of manufacturing a catalyst for purifyingexhaust gas according to the present invention provides a catalyst forpurifying exhaust gas having high durability and efficiency.

What is claimed is:
 1. A method of manufacturing a catalyst forpurifying exhaust gas, the catalyst comprising a pentasil-typecrystalline aluminosilicate catalyst carrier carrying thereon at least acopper component and a phosphorus component, the method beingcharacterized by adding the catalyst carrier into a solution containingat least a copper component, a phosphorus component, ammonia, and abuffer and having a pH of 8.0 or more, and subsequently adding an acidto the solution so as to adjust the pH of the solution to 7.0 or less,to thereby incorporate at least the copper component and the phosphoruscomponent into the catalyst carrier.
 2. A method of manufacturing acatalyst for purifying exhaust gas according to claim 1, wherein the pHof the solution before the catalyst carrier is added is 8.0-12.0.
 3. Amethod of manufacturing a catalyst for purifying exhaust gas, thecatalyst comprising a pentasil-type crystalline aluminosilicate catalystcarrier carrying thereon at least a copper component and a phosphoruscomponent, the method being characterized by adding the catalyst carrierinto a solution containing at least a copper component, a phosphoruscomponent, and ammonia, and having a pH of 9.0 or more, and subsequentlyadding an acid to the resultant solution so as to adjust the pH of thesolution to 7.0 or less, to thereby incorporate at least the coppercomponent and the phosphorus component into the catalyst carrier.
 4. Amethod of manufacturing a catalyst for purifying exhaust gas accordingto claim 1, wherein the pH of the solution before the catalyst carrieris added is 9.0-12.0.
 5. A method of manufacturing a catalyst forpurifying exhaust gas according to claim 1, wherein pentasil-typecrystalline aluminosilicate has an MFI structure.
 6. A method ofmanufacturing a catalyst for purifying exhaust gas according to claim 1,wherein the buffer is at least one species selected from among ammoniumnitrate, ammonium chloride, ammonium acetate, and sodium acetate.
 7. Amethod of manufacturing a catalyst for purifying exhaust gas accordingto claim 1, wherein the acid is at least one species selected from amongnitric acid, hydrochloric acid, and sulfuric acid.